1. Field of the Invention
The present invention relates to an image sensor having a photo diode and a method for manufacturing the same, and more particularly, to an image sensor having a photo diode for improving sensibility, junction leakage, and electron capacity, and a method for manufacturing the image sensor.
2. Description of the Related Art
A pinned photo diode is used for a complementary metal-oxide semiconductor (CMOS) image sensor, which is manufactured by CMOS processes, or a charge coupled device (CCD) image sensor to detect light for generating and accumulating photo electrodes. Since the pinned photo diode is formed in a PNP or NPN junction structure buried in a substrate, the pinned photo diode is referred to as a buried photo diode. The CMOS image sensor is subject to less power consumption than the CCD image sensor and is manufactured by a simpler process. Moreover, the CMOS image sensor can be formed together with a signal processing circuit in one chip, making it attractive as a next-generation image sensor.
The CMOS image sensor having the above-described pinned photo diode will be briefly described with reference to FIGS. 1 and 2.
FIG. 1 is a circuit diagram of a unit pixel Pix in a conventional image sensor, made up of one photo diode PD and four MOS transistors. The source (or drain) of a transfer transistor Tx is connected to the photo diode PD, and the source of a reset transistor Rx is connected to the drain (or source) of the transfer transistor Tx. A floating-diffusion capacitor Cfd is formed between the drain (or source) of the transfer transistor Tx and the source of the reset transistor Rx. The gate of a drive transistor MD is connected to the source of the reset transistor Rx and the drain (or source) of a select gate Sx is connected to the source of the drive transistor MD. In this case, a source voltage VDD is supplied to the drains of the reset transistor Rx and the drive transistor MD. A load transistor Vb is connected to the source (or drain) of the select gate Sx outside the unit pixel Pix, and the source (or drain) of the select gate Sx operates as the output of the image sensor.
FIG. 2 illustrates a semiconductor substrate in which the unit pixel of the above-described image sensor is integrated. In FIG. 2, only the photo diode, the transfer transistor, and the reset transistor are illustrated.
As shown in FIG. 2, an isolation layer 11 is formed on the substrate 10 by a conventional method. After a gate oxide layer 12 and a conductive layer 14 are deposited on the semiconductor substrate 10, portions of the layers 12 and 14 are patterned to form a transfer gate Tg and a reset gate Rg.
P-type impurities, e.g., boron ions, are implanted into the drain region (or the source region) of the transfer transistor, which is at one side of the transfer transistor, forming a p-type photo diode region 15. Then, n-type impurities, e.g., group V impurity ions such as arsenic or phosphorus ions, are implanted into a lower portion of the p-type photo diode region 15, forming an n-type photo diode region 20. As a result, p-type and n-type photo diodes are completed. Next, spacers 22 are formed on both walls of the transfer gate Tg and the reset gate Rg by a conventional blanket etching. By implanting the n-type impurities into both sides of the spacers 22, a common source region 24 and a drain region 26 of the reset transistor are formed.
However, forming the conventional n-type photo diode region 20 by implanting a single type of impurities, such as the arsenic or phosphorus ions, causes the following problems.
When the n-type photo diode region 20 is formed by implanting arsenic ions, a depletion distance becomes smaller because the projection distance ΔRp and diffusivity of the arsenic ions are small. Consequently, a high concentration of arsenic ions is maintained in each unit area, increasing junction capacitance. As a result, the electron capacity of the photo diode is improved. However, since the projection distance ΔRp and diffusivity of the arsenic ions are small, it is difficult to form the n-type photo diode region 20 over a large area. Accordingly, the sensitivity of the photo diode is reduced, and it is likely that junction leakage occurs with an abrupt junction profile.
When the n-type photo diode 20 is formed by implanting phosphorus ions, the photo diode is easily formed over a large area, due to a large projection distance ΔRp and diffusivity, improving sensitivity. In addition, the junction leakage is reduced by smoothing the profile on a junction interface. However, since the projection distance ΔRp and diffusivity of the phosphorus ions are large, the depletion distance is increased, thereby reducing the junction capacitance and the concentration of phosphorus ions in each unit area. As a result, the electron capacity of the photo diode is reduced.